"This system is especially fascinating because no model of either planet or star formation fully explains what we see," study lead researcher Vanessa Bailey, a fifth-year graduate student in the University of Arizona's department of astronomy, said in a statement.

Most planets are formed when an asteroid-like body accumulates gas and dust from its host star, but in this case, the alien planet is too far away to do that. Bailey says an alternate theory of planet formation involving mini binary star systems could account for the formation of HD 106906 b.

"A binary star system can be formed when two adjacent clumps of gas collapse more or less independently to form stars, and these stars are close enough to each other to exert a mutual gravitation attraction and bind them together in an orbit," she explained.

But in instances like that, the mass ratio between the two stars is normally no more than 10 to 1.

"In our case, the mass ratio is more than 100-to-1," Bailey said. "This extreme mass ratio is not predicted from binary star formation theories — just like planet formation theory predicts that we cannot form planets so far from the host star."

Astronomers plan to further study the alien planet to find out more about how it formed and why it's so large.

"Systems like this one, where we have additional information about the environment in which the planet resides, have the potential to help us disentangle the various formation models," Bailey said. "Future observations of the planet's orbital motion and the primary star's debris disk may help answer that question."